Typical x-ray tubes generate x-ray radiation over a relatively wide solid angle. To avoid unnecessary exposure to both the patient and the medical team, collimators of x-ray absorbing materials such as lead are used to block the redundant radiation. This way only the necessary solid angle of useful radiation exits the x-ray tube to expose only the necessary elements.
Such collimators may assume a variety of designs and x-ray radiation geometry. Collimators can be set up manually or automatically using as input, for example, the dimensions of the cassette holding the plate to limit the radiation to the dimensions of the film in the cassette.
In fluoroscopy the situation is more dynamic than in a single exposure x-ray. The x-ray radiation is active for long periods and the treating physician typically has to stand near the patient, therefore near the x-ray radiation. As a result, it is desired to provide methods to minimize exposure to the medical team. Methods for reducing x-ray radiation intensity have been suggested where the resultant reduced signal to noise ratio (S/N) of the x-ray image is compensated by real-time digital image enhancement. Other methods suggest a collimator limiting the solid angle of the x-ray radiation to a fraction of the image intensifier area and moving the collimator to sweep the entire input area of the image intensifier where the Region of Interest (ROI) is exposed more than the rest of the area. This way, the ROI gets high enough x-ray radiation to generate a good S/N image while the rest of the image is exposed with low x-ray intensity, providing a relatively low S/N image. The ROI size and position can be determined in a plurality of methods. For example, it can be a fixed area in the center of the image or it can be centered automatically about the most active area in the image, this activity is determined by temporal image analysis of a sequence of cine images received from the video camera of the fluoroscopic system.
Reference is made now to FIG. 1A which presents a typical layout of a fluoroscopy clinical environment.
X-ray tube 100 generates x-ray radiation 102 directed upward occupying a relatively large solid angle towards collimator 104. Collimator 104 blocks a part of the radiation allowing a smaller solid angle of radiation to continue in the upward direction, go through bed 108 that is typically made of material that is relatively transparent to x-ray radiation and through patient 110 who is lying on bed 108. Part of the radiation is absorbed and scattered by the patient and the remaining radiation arrives at the typically round input area 112 of image intensifier 114. The input area of the image intensifier is typically in the order of 300 mm in diameter but may vary per the model and the technology. The image generated by image intensifier 114 is captured by video camera 116 and then displayed on monitor 118 as image 120.
In modern systems the image intensifier and video camera are often replaced by a rectangle flat panel detector. It would be appreciated that the description below referring to image intensifiers and video cameras is analog for the case of a flat panel detector or other detectors converting x-ray radiation to an electronic image.
Operator 122 is standing by the patient to perform the medical procedure while watching image 120.
The operator has a foot-switch 124. When pressing the switch, continuous x-ray radiation is emitted to provide cine imaging 120. The intensity of x-ray radiation is typically optimized in a tradeoff of low x-ray intensity that is desired to reduce exposure to the patient and the operator and high x-ray intensity that is desired to enable a high quality image 120 (high S/N). With low intensity x-ray radiation and thus low exposure of the image intensifier input area, the S/N of image 120 might be so low that image 120 becomes useless.